The present invention relates to an image information processing apparatus and, more particularly, to an image information signal processing apparatus which is used for displaying, with high quality, different types of picture images read by an image sensor from a document.
Recently, image processing systems have been strongly required to deal with a variety of types of office documents, which include color graphics images in addition to normal character/line-based documents. Such graphics images may contain continuous-tone picture images, such as photographs, and pseudo continuous-tone picture images, such as newspaper dot-photographs that are constructed by arranging various sizes of dots of three primary colors so as to provide a coarse photographic image which tries to "stimulate" the original natural photograph.
The "bi-level" images may be defined as images of two distinct density levels, which typically include the character/line-based document images. The pseudo continuous-tone picture images may also be categorized in the bi-level images. These pseudo continuous-tone picture images are sometimes called "dot-pattern" picture images, "dot photograph" picture images, "halftone" images, or "digitized" photograph images.
With presently available image output devices such as liquid-crystal display units or thermal printers, bi-level images can be displayed very stably. Where a general image which contains a continuous-tone picture image such as a photograph image is displayed or printed by these output devices, a continuous-tone image display/print scheme using a dithering technique is conventionally used, wherein a threshold value used for bi-level quantization is varied in accordance with a random or pseudo random function during the continuous-tone picture image information is being quantized into bi-level information.
With such a technique, however, when an image is displayed or printed with a number of gradation, not only does the resolution decrease but irregular noise is liable to be produced. This results in the image quality of an input image containing various types of image components being degraded.
An apparatus is known, which is arranged to input an image using a color image sensor equipped with spatially-divided color filters and output an output signal from the color image sensor to a color printer to reproduce the image. When an input document image containing a color continuous-tone picture image is copied by such an apparatus, dots forming the continuous-tone picture image are reproduced with increased noises, which results in that the colors in the printed document cannot be reproduced precisely.
An improved image information processing system has been disclosed in a copending U.S. patent application Ser. No. 556,323 filed Nov. 30, 1983 (U.S. Pat. No. 4,663,662 issued May 5, 1987); entitled "Picture Signal Processing System Suitable For Displaying Continuous Tone Pictures" and assigned to the same assignee as this patent application. According to one of the methods which are disclosed in this application, the local density variation of an input image is detected. If the density variation is greater than a predetermined specific value, an image information processing circuit quantizes an input image signal into a bi-level image signal on the basis of a fixed threshold value. On the other hand, if the local density variation is smaller than the specific value, the circuit dithers the input image signal.
According to the other method of the patent application, a brightness signal and color information signals are averaged in order to display a color pseudo continuous-tone picture image being constituted by various sizes of dots in three primary colors. By averaging the color information signals, dot-noises can be eliminated. The local density variation of the input image is detected on the basis of the brightness signal that has the dot-noises eliminated therefrom. If this density variation is greater than a preselected specific value, the color information signals are subjected to bi-level quantization using a fixed threshold value. If the density variation is smaller than the specific value, the color information signals are subjected to the dither-processing.
Unfortunately, both of the above methods fail to provide successful reproduction of various types of image images, since a high grade of technical difficulty has been laid in precise detection of pseudo continuous-tone picture regions from within the input image, as well as in selection of the most suitable type of image processing for the detected image regions.